Aquareovirus NS31 protein serves as a specific inducer for host heat shock 70-kDa protein

Recognition of novel proteins encoded by an aquareovirus using mass spectrometry

AQUAREOVIRUS: a genus of within the family Spinareoviridae, order Reovirales, infects aquatic animals. Their genomes comprise 11 segments of double-stranded RNA, which function directly as mRNAs upon release into the cytoplasm of infected cells. Here, liquid chromatography-tandem mass spectrometry was employed to annotate small coding ORFs in the Aquareovirus-C genome. Its plus-strand RNA of segment 8 (S8) contains a novel protein-coding frame (NS15), and S5 seems to has an additional reading frame (NS18) with a putative non-AUG initiation codon. Among them, NS15 polypeptide has been proved by immunoblotting assay. Remarkably, the S4 and S11 minus-strand mRNAs may encode polypeptides, suggesting ambisense polarity of the two segmented RNAs. And the newly discovered NS12 ORF in 2019, from viral tricistronic S7 mRNA, was also confirmed by this mass-spectrometry data. Taken together, these identified new ORFs reveal the genome-coding complexity of Aquareovirus-C.

Involvement of transcriptional co-activator p300 in upregulated expression of HSP70 by aquareovirus non-structural protein NS31

Members of Aquareovirus genus, including grass carp reovirus (GCRV), contribute to a serious threat to aquaculture animals accompanied by stress response. Our previous reports revealed that GCRV nonstructural protein NS31 serves as a potent contributor for virus selectively up-regulating specific heat shock protein 70-kd gene（HSP70），however，the mechanism by which inducing HSP70 gene expression is unknown. In this study, we further found that either the N- or C-terminal domain of GCRV NS31 is responsible for enhancing fish HSP70 promoter transcription, and recombinant NS31 protein purified from baculovirus expression system seems to not directly bind HSP70 basic promoter in vitro by an electrophoretic mobility shift assay. However, the transcriptional co-activator p300 was identified as a potential interacting partner for NS31 by pull-down assay. Moreover, knock-down of p300 or addition of p300 inhibitor resulted in obviously reduced HSP70 expression by NS31 or GCRV infection suggesting that the well-characterized heat-shock-responsive HSF1/p300 transcriptional complex might involve in the induction of HSP70. These results collectively reveal this aquareovirus generates cell stress response through its nonstructural protein NS31 recruiting transcriptional co-activator p300.

Nonstructural protein NS17 of grass carp reovirus Honghu strain promotes virus infection by mediating cell-cell fusion and apoptosis

Fusion-associated small transmembrane (FAST) proteins can promote cell fusion, alter membrane permeability and trigger apoptosis to promote virus proliferation in orthoreoviruses. However, it is unknown whether FAST proteins perform these functions in aquareoviruses (AqRVs). Non-structural protein 17 (NS17) carried by grass carp reovirus Honghu strain (GCRV-HH196) belongs to the FAST protein family, and we preliminarily explored its relevance to virus infection. NS17 has similar domains to FAST protein NS16 of GCRV-873, comprising a transmembrane domain, a polybasic cluster, a hydrophobic patch and a polyproline motif. It was observed in the cytoplasm and the cell membrane. Overexpression of NS17 enhanced the efficiency of cell-cell fusion induced by GCRV-HH196 and promoted virus replication. Overexpression of NS17 also led to DNA fragmentation and reactive oxygen species (ROS) accumulation, and it triggered apoptosis. The findings illuminate the functions of NS17 in GCRV infection, and provide a reference for the development of novel antiviral strategies.

Cytokine-like activity of European sea bass ISG15 protein on RGNNV-infected E-11 cells

IFN-I generates an antiviral state by inducing the expression of numerous genes, called IFN-stimulated genes, ISGs, including ISG15, which is the only ISG with cytokine-like activity. In a previous study, we developed the Dl_ISG15_E11 cell line, which consisted of E11 cells able to express and secrete sea bass ISG15. The current study is a step forward, analysing the effect of secreted sea bass ISG15 on RGNNV replication in E11 cells, and looking into its immunomodulatory activity in order to corroborate its cytokine-like activity. The medium from ISG15-produccing cells compromised RGNNV replication, as it has been demonstrated both, by reduction in the viral genome synthesis and, specially, in the yield of infective viral particles. The implication of sea bass ISG15 in this protection has been demonstrated by ISG15 removal, which decreased the percentage of surviving cells upon viral infection, and by incubation of RGNNV-infected cells with a recombinant sea bass ISG15 protein, which resulted in almost full protection. Furthermore, the immunomodulatory activity of extracellular sea bass ISG15 has been demonstrated, which reaffirms a cytokine-like role for this protein.

Multiple isoforms of HSP70 and HSP90 required for betanodavirus multiplication in medaka cells

Heat shock proteins (HSPs) are molecular chaperones that have recently been shown to function as host factors (HFs) for virus multiplication in fish as well as in mammals, plants, and insects. HSPs are classified into families, and each family has multiple isoforms. However, no comprehensive studies have been performed to clarify the biological importance of these multiple isoforms for fish virus multiplication. Betanodaviruses are the causative agents of viral nervous necrosis in cultured marine fish and cause very high mortality. Although the viral genome and encoded proteins have been characterized extensively, information on HFs for these viruses is limited. In this study, therefore, we focused on the HSP70 and HSP90 families to examine the importance of their isoforms for betanodavirus multiplication. We found that HSP inhibitors (17-AAG, radicicol, and quercetin) suppressed viral RNA replication and production of progeny virus in infected medaka (Oryzias latipes) cells. Thermal stress or virus infection resulted in increased expression of some isoform genes and facilitated virus multiplication. Furthermore, overexpression and knockdown of some isoform genes revealed that the isoforms HSP70-1, HSP70-2, HSP70-5, HSP90-α1, HSP90-α2, and HSP90-β play positive roles in virus multiplication in medaka. Collectively, these results suggest that multiple isoforms of fish HPSs serve as HFs for betanodavirus multiplication.